As some of you know in the past when we took the bus out the brakes after an hour would lock up when the air pressure dropped. Dallas told me to dump in a few bottles of Alcohol, into the compressor supply line to get it home which did work but not good for rubber parts.So off and on for 2 years when I had time I separated the bag system which works well now at all 4 corners.Bags stays up as long as I want it too 80 to 90 lbs and holds until I let it out.

Dallas also mentioned it might be the compressor going or check all the valves for leaks etc. I did and replaced;GovenorChange wipers to electric.R-8 Relay valve.All Air regulatorsPPV (65 PSI pressure protection valve)3 check valves rebuilt Air Dryer.Inversion valve.2 way check valve in rear.check all 4 tanks which are in good condition.Installed new chain drains at tanks.E-Type FilterSafety valveUnloader piston kit at compressorDisconnected Leveling valves, don't need them anymore.Am checking brake diaphragms and so far they are goodDo not hear any leaks or show any bubbles.

The system builds up fine but when I tap down the service brake to 80 it builds up to 90 in 1 minute or less. After i do that for a few times it will finally climb to 118 which is what my book says to set it at.When I shut it off the air will hold at 118 for 30 minutes and then slowly drop to 80 in an hour then down to 0 in 2 hours.

Haven't had it on the road for awhile until I am sure this is right.

I am avoiding changing the compressor (not the money just PITA on a 5 MCI) but that is probably what it is. Any Ideas or just change it out?

I am not sure what to say about the compressor and assume you have already cleaned/changed the governor. It sounds to me, when you say the system leaks down to about 80psi when you shut it down, that it is leaking off the accessory tank first, and then more slowly off the main system. Mine does that too. I am pretty sure that it is my windshield wiper valves that leak a little. My pressure gauge comes off the accessory tank, so it will read zero. However, I have another gauge in the back that will hold at 85 psi when that happens.

Lin, yes also change governor, electric wipers. Where did you put the other gauge in the back? Leaks to 80 but then all the way down in a short time. The next day I check the wet tank and it is empty and the park tank has a little in it but not much. Dry tank is empty. It has to be leaking out somewhere or internally somewhere.

The gauge on a 5C reads the dry tank, which is the main service tank. The recovery test, which is fan down to 80 psi then time from 85 psi to 100 psi after the system is fully up and stable, should take about 15 - 20 seconds at high idle (1,000 rpm or so) on a 5C. Longer than that indicates a failing or worn compressor, even if the DOT test allows up to 2 minutes for that test. The compressor should always raise the air pressure steadily, at an even rate of climb, to 120 psi or the cut-off pressure and signal the air dryer to purge. Not climbing steadily indicates a compressor problem. Climbing then stopping without purging indicates a compressor problem as opposed to a possible governor problem.

For me, you have at least three symptoms indicating a compressor problem. Given the rebuild you've given to the rest of the system I'd hesitate to point the finger elsewhere at this point, so I think the compressor needs work. The leaking is interesting. My system normally falls to 60 psi overnight, falls to zero over the next day, but the parking tank holds air almost forever. The check valves at the entry to the parking tank, between the dry tank and the wet tank, and the air dryer are supposed to isolate those systems from each other.

The discharge valve inside the compressor can leak allowing air to escape into the cavity of the compressor it is almost impossible to detect your problem I would not worry about it, making good air pressure with no oil or water in the tank run it

Brian, That is the only check valve i haven't replaced yet. It is at the top of the tank and i can undo the tank straps but it has a 3way net to attached to the check valve and is almost impossible to get out without cutting all the lines of taking up the flooring in the front hall so have been thinking about how to get to it. That maybe why it is draining the park tank over night.

Clifford ya I know get it on the road!... LOL i would like to but since It is parked and the wife still can't climb the step for awhile I want it as close to right as i can get it. then I can work on the inside and finish up a lot of loose ends in there.